And then there’s the main reason: Russia would not be assured of victory. The Ukrainian Army is much improved, having upscaled its equipment and preparations for a ground invasion, and the Russian troops deployed near the border are most likely insufficient to conquer the country. Because of its sheer bulk, the Russian Army might be able to advance: Quantity has a quality of its own, as Stalin reportedly said. But it would surely come at the cost of catastrophic losses in human life.

If he had little intention of invading, why did Mr. Putin raise the stakes so high? The answer is simple: Afghanistan. The West’s disastrous withdrawal from the country in August signaled the United States’ waning appetite for entanglement abroad. Emboldened, Mr. Putin clearly decided it was a good time to press his case for a revision of the post-Cold War order. Without the usual bargaining chips — no sound economy, no superior weapons, no fanatical followers — he fell back on unpredictability. The more irrational his behavior, went the thinking, the more likely the United States would accept his demands.

Those demands, published in mock-treaty form in December, were in many cases absurd. The call for NATO to withdraw its troops from members in Eastern Europe, for example, would never be met. The core request — that NATO deny membership to Ukraine — was silly in a different way. There was no chance of Ukraine becoming a member any time soon, ultimatum or not. But that was Mr. Putin’s point: By demanding something that was already happening, Mr. Putin aimed to claim a victory over the West.